This invention is preferably to be used with radar apparatus employing pulse compression techniques, however, the disclosed filter may be advantageously utilized in any signal processing environment wherein an output signal includes a desired spike or pulse of information sandwiched among numerous sidelobes each having the same period. The disclosed filter may be utilized to reduce the magnitude of such sidelobes when such sidelobes normally have no more than slight pulse-to-pulse variations in amplitude.
In radar systems, pulse compression techniques are utilized to put sufficient energy upon a target so that it may be recognized at longer ranges (or so that smaller targets may be recognized) than would otherwise be possible without sacrificing range and Doppler information from analysis of the reflected echo signal. Pulse compression involves the transmission of a long coded pulse and the processing of the received echo to obtain a relatively narrow pulse. Thus, increased detection capability of a long pulse radar is realized while retaining the range resolution of a narrow pulse system. The received echo is processed in the receiver by a matched filter. The matched filter readjusts the relative phases of the frequency components so that a narrow or compressed pulse is again produced. The matched filter results in a correlation of the received signal with the transmitted signal, and thus the output of the matched filter is referred to as the correlation output signal.
As is well known in the pulse compression art, the output of a matched filter typically includes numerous sidelobes in addition to the desired narrow or compressed pulse. Since the sidelobes can be interpreted by a radar system as echoes from that which is, of course, one or more nonexistent targets, it is desirable to select pulse compression schemes which minimize sidelobes and/or use filters to reduce such sidelobes. In the prior art, such sidelobes were reduced by appropriate amplitude weightings to reduce the sidelobes. Examples of such weighting functions are Taylor and Hamming weightings. However, employing such weighting functions is typically both complex and costly, especially in the presence of an expected Doppler frequency shift, where a bank of matched filters is utilized with each filter being matched with different frequencies so as to cover the band of expected Doppler frequencies.
It was, therefore, one object of this invention that the sidelobe response of the auto-correlation output signal be reduced using relatively simple and inexpensive circuit components.
There is yet another object of this invention, that the sidelobe rejection filter be implementable using either analog or digital circuit technology.
The aforegoing objects are achieved as is now described. The auto-correlation output signal, assuming that it has a comparatively narrow pulse and a plurality of sidelobe pulses, each of the pulses having a uniform period .tau. and the relative variations in the amplitude of adjacent sidelobe pulses being relatively minor, is first delayed by .tau.. The output signal is then applied in both .tau. delayed and undelayed forms to a difference circuit for determining between .tau. delayed and undelayed signals. If the pulse compression technique being used is linear FM, the output of the difference circuit is rectified and the rectified output signal will include the desirable narrow pulse with a relatively small amount of sidelobe information. If the pulse compression technique being used utilizes Barker Codes for phase coding the compressed signal, then further filtering operations may be used, if desired, to further reduce sidelobe response.
In the case of a Barker phase coded signal, the output of the difference circuit is preferably applied to two rectification or polarity determining circuits respectively picking off the positive and negative portions of the output of the difference circuit. One of the outputs of the rectification circuits is thence delayed by N.tau. wherein N is the number of bits in the Barker code being utilized. The output of the N.tau. delay circuit and the output from the other rectification circuit is thence added and rectified to provide an output signal substantially free of sidelobe responses.
The advantage of the invention, both as to its construction and mode of operation, and the preferred mode of use, will be readily appreciated by those skilled in the art referring to the following detailed description of an illustrative embodiment when considered in conjunction with the accompanying drawings in which like reference numerals refer to the same component throughout the drawings.